feat: add WSL environment hint to system prompt (#8285)

When running inside WSL (Windows Subsystem for Linux), inject a hint into
the system prompt explaining that the Windows host filesystem is mounted
at /mnt/c/, /mnt/d/, etc. This lets the agent naturally translate Windows
paths (Desktop, Documents) to their /mnt/ equivalents without the user
needing to configure anything.

Uses the existing is_wsl() detection from hermes_constants (cached,
checks /proc/version for 'microsoft'). Adds build_environment_hints()
in prompt_builder.py — extensible for Termux, Docker, etc. later.

Closes the UX gap where WSL users had to manually explain path
translation to the agent every session.

agent/prompt_builder.py

@@ -12,7 +12,7 @@ import threading
 from collections import OrderedDict
 from pathlib import Path
 
-from hermes_constants import get_hermes_home, get_skills_dir
+from hermes_constants import get_hermes_home, get_skills_dir, is_wsl
 from typing import Optional
 
 from agent.skill_utils import (
@@ -366,6 +366,36 @@ PLATFORM_HINTS = {
     ),
 }
 
+# ---------------------------------------------------------------------------
+# Environment hints — execution-environment awareness for the agent.
+# Unlike PLATFORM_HINTS (which describe the messaging channel), these describe
+# the machine/OS the agent's tools actually run on.
+# ---------------------------------------------------------------------------
+
+WSL_ENVIRONMENT_HINT = (
+    "You are running inside WSL (Windows Subsystem for Linux). "
+    "The Windows host filesystem is mounted under /mnt/ — "
+    "/mnt/c/ is the C: drive, /mnt/d/ is D:, etc. "
+    "The user's Windows files are typically at "
+    "/mnt/c/Users/<username>/Desktop/, Documents/, Downloads/, etc. "
+    "When the user references Windows paths or desktop files, translate "
+    "to the /mnt/c/ equivalent. You can list /mnt/c/Users/ to discover "
+    "the Windows username if needed."
+)
+
+
+def build_environment_hints() -> str:
+    """Return environment-specific guidance for the system prompt.
+
+    Detects WSL, and can be extended for Termux, Docker, etc.
+    Returns an empty string when no special environment is detected.
+    """
+    hints: list[str] = []
+    if is_wsl():
+        hints.append(WSL_ENVIRONMENT_HINT)
+    return "\n\n".join(hints)
+
+
 CONTEXT_FILE_MAX_CHARS = 20_000
 CONTEXT_TRUNCATE_HEAD_RATIO = 0.7
 CONTEXT_TRUNCATE_TAIL_RATIO = 0.2